Using concrete blocks instead of milk crates

That poor fishy stuck in the milk crate has got me think'in. I am using
milk crates for plant stands, my fish are still fairly small, less than 10
inches, someone said they use concrete blocks treated or acid washed or
painted or something. Exactly what did you do to them to prevent leeching
lime "I think it's lime" I thought about painting them black but with what
kind of paint.....Thoughts...I don't want my fishies stuck in a milk crate
hole...or...is there an alternative to concrete....Thanks Claude